Eastern Pacific region - Marine zoogeographic regions fishes

The eastern Pacific region contains another radiation related to and only recently separated from the western Atlantic.

Eastern
Pacific region

The
eastern Pacific region contains another radiation related to and only recently
separated from the western Atlantic. The region contains only four to eight
genera of zooxanthellate corals (Rosen 1988) and fewer species of fishes than
are present in the western Atlantic. Some widespread taxa, such as the Bluefish
(Pomatomus saltatrix) and Cobia (Rachycentron canadum), are
absent. The eastern Pacific barrier, the huge expanse of open water
between the central and south Pacific islands and the American mainland, acts
as a distance barrier limiting the movement of 86% of shore species from the
central Pacific (Briggs 1974).

Elevation
of the Panamanian Isthmus approximately 3 million years ago separated the
continuous distribution of species into eastern Pacific and western Atlantic
populations. David Starr Jordan, pioneer American ichthyologist, referred to
such pairs of species as geminate species, related species divided by
the isthmus, such as the Spanish mackerels, Scomberomorus sierra, in the
eastern Pacific, and S. brasiliensis, in the Caribbean Sea. Some
geminate species have clearly differentiated into what can be called good
species from a morphological and sometimes from a genetic point of view as
well, such as Spanish mackerels and toadfishes of the genus Batrachoides.
Others, such as the halfbeaks of the genus Hyporhamphus are less well
differentiated morphologically, making molecular methods useful to reach
decisions on the status of the populations on either side of the isthmus (see The Panama barrier).

The
completeness of the eastern Pacific barrier is, however, not quite as distinct
as Briggs (1974) implied (Lessios & Robertson 2006). Several Indo-West Pacific
shore fishes actually cross the eastern Pacific barrier and are found at
offshore islands such as the Revilligedos off the coast of Mexico, and
Clipperton and Cocos off the coast of Costa Rica. Distributions of a species of
mackerel (Scomber australasicus) and a needlefish (Tylosurus acus
melanotus) extend from the western Pacific through the Hawaiian Islands to
these islands, but these species are replaced by related forms (S. japonicus
and T. Pacificus, respectively) along the eastern Pacific coast of
Middle America. These exceptions to the completeness of the eastern Pacific
barrier may be related to habitat differences between the offshore islands and
the mainland.

The
Panama Canal connects the eastern Pacific with the western Atlantic. However,
unlike the Suez Canal, the Panama Canal is not at sea level. It contains a
freshwater lake, Lake Gatun, in its middle, and fresh water is used to raise
the water level in a series of locks to lift ships up to the lake and then down
to the ocean on the other side. This freshwater barrier prevents marine species
from moving between the two oceans, with the exception of a few species that
tolerate a wide range of salinities (McCosker & Dawson 1975). A proposed
sea-level canal would allow mixing of the two different faunas and might have
grave effects on the fishes and marine invertebrates on both sides of the
isthmus. Diseases, parasites, and aggressive Indo-West Pacific species that
pose little current danger in the eastern Pacific, such as the crown-of-thorns
starfish and a sea snake, might do severe damage to coral reefs and the fish
fauna of the western Atlantic (Briggs 1974).